Field of the Invention
The present invention relates to an organic light emitting diode display.
Discussion of the Related Art
An active matrix organic light emitting diode display includes organic light emitting diodes (OLEDs) capable of emitting light by itself and has advantages of a fast response time, a high emission efficiency, a high luminance, a wide viewing angle, and the like.
The OLED serving as a self-emitting element includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emission layer EML and form excitons. As a result, the emission layer EML generates visible light.
The organic light emitting diode display arranges pixels each including the OLED in a matrix form and adjusts a luminance of the pixels based on gray levels of video data. As shown in FIG. 1, each pixel may include a driving thin film transistor (TFT) DT controlling a driving current flowing in the OLED, a first switching TFT ST1 which is turned on in response to a first gate pulse SCAN and applies a data voltage Vdata to a gate node Ng of the driving TFT DT, a second switching TFT ST2 which is turned on in response to a second gate pulse SEN and applies a reference voltage VREF to a source node Ns of the driving TFT DT, and a storage capacitor Cst for holding a gate-to-source voltage Vgs of the driving TFT DT for a predetermined period of time. The driving TFT DT controls a magnitude of the driving current supplied to the OLED depending on a magnitude of the voltage Vgs stored in the storage capacitor Cst and adjusts an amount of light emitted by the OLED. The amount of light emitted by the OLED is proportional to a current supplied from the driving TFT DT.
The data voltage Vdata applied to the gate node Ng of the driving TFT DT varies depending on data of an input image, but the reference voltage VREF applied to the source node Ns of the driving TFT DT is applied to all of the pixels at a fixed value irrespective of the input image as shown in FIG. 2. The reference voltage VREF generally uses a voltage greater than 0V, so as to prepare for case where a threshold voltage of the driving TFT DT is negatively shifted. Thus, as shown in FIG. 3, because the gate-to-source voltage Vgs of the driving TFT DT defining a gray-level representation region is less than the maximum data voltage Vdata, it is impossible to implement a luminance corresponding to the maximum data voltage Vdata. This reduces the gray-level representation and leads to a reduction in image quality. For example, FIG. 4 shows a relationship between the gate-to-source voltage Vgs and the driving current Ids when the reference voltage VREF is fixed to 3 V.